The invention relates to a matrix display device as defined in the pre characterizing part of claim 1. Such a matrix display device is especially useful for displaying a video signal with a number of video lines which is lower than a number of display lines of a matrix display of the matrix display device.
EP-A-0565167 discloses a solution to displaying an NTSC video signal on a PAL matrix display. In this prior art, a row driver circuit drives several rows of picture elements with the same picture information to repeat certain lines of the NTSC video signal. In this way, the picture is effectively expanded in the vertical direction to fill the available display area. However, this technique may lead to perceivable display artifacts. Differences in the picture element voltages for repeated and non-repeated lines may occur.
It is, inter alia, an object of the invention to provide a solution to expanding a video signal with a number of scanning lines which is smaller than the number of display lines of the matrix display with fewer artifacts.
The matrix display device according to the invention comprises a matrix display with picture elements arranged in a predetermined number of display lines. A driving circuit supplies picture signals to the picture elements, a timing circuit generates select periods, and a selecting circuit selects the display lines.
The driving circuit receives a video signal which comprises a number of video lines which is lower than the number of display lines of the matrix display, and supplies picture data signals which are dependent on an associated one of the video lines to picture elements of a selected one of the display lines. The timing circuit receives video timing information to supply consecutive and non-overlapping select periods. Each select period occurs completely within a line period which has the duration of one of the video lines. This implies that the select periods are locked to a repetition period of the video lines. The video timing information may comprise line and field synchronizing pulses of the video signal. The selecting circuit generates consecutive select pulses to successively select the display lines during the consecutive select periods.
As the number of video lines is lower than the number of display lines of the matrix display, the matrix display device has to generate extra video lines. The extra video lines are generated during certain line periods which, for the sake of clarity, are further referred to as repeating line periods. During these repeating line periods, at least two select periods are present for selecting successively at least two corresponding display lines, both of which receive video information. In the other line periods, which are referred to as normal line periods, the video information of a line period is displayed on the corresponding display line.
In the prior art, the select period during a normal line period is substantially equal to the line period, while in repeating line periods in which two select periods occur, the select periods are substantially equal to half the line period. These different select times cause striping artifacts. The invention reduces these striping artifacts to a large extent by selecting the select periods during the normal line periods to be equal to the select periods as occur during the repeating line periods.
For example, in one embodiment of the invention, after a certain number of consecutive normal video line periods, during a repeating line period, two consecutive select periods are generated to display video data signals on two consecutive display lines. The two select periods fit within the duration of one line period of the video signal. Consequently, all display lines are selected during a select period which is smaller than half the line period. In a practical case, wherein an expansion factor of 4:3 is required, the timing circuit generates a repeating line period after every two consecutive normal line periods. In this way, the repeating line periods are evenly distributed across the display lines, and only one line of extra video information has to be generated during the repeating line period.
In another embodiment of the invention, the extra video information is obtained in a simple way by repeating the video information of a line period. In this way, the same video information is displayed on the two consecutive display lines.
In yet another embodiment of the invention, the extra video information depends on the video information of more than one line period. For example, the extra video information may be interpolated from the video information of two adjacent line periods.
According to EP-A-0565167, the artifacts of the prior art caused by non-equal select times are minimized by controlling a row driver circuit in such a way that it scans the rows of picture elements in turn at a rate which is a function of the number of rows in a panel and the field period of the applied video signal. In this way, all the rows (the display lines) are addressed within one field period of the video signal with an equal addressing period (the select period). It should be noted that the row driver circuit is operated in an asynchronous manner: the timing of the row driver circuit is not directly linked with the timing of the video lines. Consequently, the select periods do not occur completely within one of the line periods. This results in a complicated driving circuit. Furthermore, the asynchronous display of the video signal may also cause artifacts. Thus, although EP-A-0565167 discloses the problem of the prior art, a totally different solution to obtaining select periods all having the same duration is disclosed when compared with the solution provided by the present invention.
EP-A-0794524 discloses a matrix display with aspect ratio conversion. The matrix display is a liquid crystal display (LCD) with an aspect ratio of 16:9 and 240 display lines. If an EDTV2 (Extended Definition Television 2) video signal with 180 scanning lines per frame is displayed on such a display, the top and bottom part of the display will not receive a video signal, as the number of scanning lines (the line periods) in the video signal is smaller than the number of select lines (the display lines) of the display. A driving circuit has been provided which writes a scanning line of the video signal simultaneously into two select lines once in every three scanning lines of the video signal. In this way, a video signal with a number of scanning lines which is smaller than the number of select lines of the LCD is vertically expanded to also display video in the top and bottom part of the display. A principal difference between this prior art and the present invention is that, according to the present invention, the select periods are non-overlapping. This prior-art solution cannot be implemented in matrix displays in which the simultaneous selection of select lines causes artifacts, or if the selecting circuit is unable to select more than one select line at a time.
Although the striping artifacts due to the different select periods of the prior art are effectively reduced in the present invention, still some annoying striping may remain visible in that the first video line in a repeating line period appears darker than the other video lines.
Another embodiment of the invention is based on the insight that this residual striping is caused by the fact that the polarity of the video lines in the repeating line period is equal, while the polarity between other display lines changes sign. Video lines which are followed by a video line with the same polarity appear darker. The difference of transmission between the display lines is compensated by adapting the drive signals in such a way that the transmission of the darker rows is increased, or that the transmission of the brighter rows is decreased.
In another embodiment of the invention, in a TFT LCD, the difference of transmission between the display lines is compensated by adapting the common signal in such a way that the transmission of the darker rows is increased, or that the transmission of the brighter rows is decreased. The common signal is supplied to the common electrode which interconnects the picture elements.
In still another embodiment of the invention, in a TFT LCD, the transmission of all display lines has been made equal by increasing the amount of kickback in positive fields for the darker rows, and by decreasing the amount of kickback in negative fields for the darker rows, or the other way around for the lighter rows. The amount of kickback may be influenced by superimposing a square-wave signal on the gate select signal of the TFTs. The kickback effect is the capacitive crosstalk from the gate select signal to the picture elements via the gate source capacitance of the TFTs.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.